1. Field of the Invention
Exemplary embodiments of the present invention relate to inorganic phosphors based on silicate compounds which convert higher-energy excitation radiation, i.e. ultraviolet (UV) or blue light, with high efficiency into a longer-wavelength radiation which may be in the visible spectral range.
2. Discussion of the Background
Phosphors may be used in light sources such as light emitting diodes (LEDs), which emit colored or white light. In an LED, phosphors, which may be used in combination with other luminophores, are used to convert ultraviolet or blue primary radiation emanating from the LED into a longer wavelength secondary radiation, in particular white light.
Although various phosphors having high luminescent output, such as cerium-doped yttrium aluminium garnets, europium-activated alkaline earth metal orthosilicates, and similarly doped nitrides of different composition, have already been described for these applications, further efforts to develop improved materials for use in LEDs are known. The development trends consist in particular in finding phosphors having improved temperature characteristics and having higher stability under the resulting radiation load and under the influence of atmospheric humidity and other environmental factors. Such luminophores may be required for the production of LED lamps having a relatively high power consumption and improved lifetime.
Europium-activated alkaline earth metal oxyorthosilicates of the general type Sr3SiO5:Eu have become known for use in LEDs which emit colored or white light. Such phosphors are described, for example, in WO 2004/085570A1 and WO 2006/081803A1 and in various scientific publications, such as “Application of Strontium Silicate Yellow Phosphor for White Light-emitting Diodes” by Park, Joung-Kyu, et al., in Appl. Phys. Lett. 84 (2004), 1647-49, and “Photoluminescence properties of Eu2+-activated Sr3SiO5 phosphors” by Jee, Soon-Duc, et al., in J. Mater. Sci. 41 (2006), 3139-41.
The known luminophores emit light in the yellow to orange range of the visible spectrum and are distinguished by high luminescent efficiencies and extremely low thermal is quenching up to temperature values of 250° C. In this respect, they are substantially superior to the orthosilicates, likewise emitting between 580 and 610 nm, as an orange component in phosphor mixtures for warm white LEDs and, because of these advantageous properties and owing to the substantially lower production costs, could compete even with the red-emitting nitride phosphors increasingly favored for these applications.
However, under specific conditions LEDs produced from such phosphors may have a comparatively short lifetime. A possible cause of this disadvantageous behavior may be the relatively high moisture sensitivity of the europium-doped alkaline earth metal oxyorthosilicates. As a result, industrial utility of these luminophores may be prevented or complicated in certain areas.